126 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



lowlands of southern Brazil, and have been described by Mitchell 

 ( I 957)- The most common birds in some of these flocks are green- 

 headed tanagers. Mitchell suggests that they play a nuclear role in 

 these flocks. Her description would imply that they show a high de- 

 gree of intraspecific gregariousness and are usually or always passive 

 nuclear in mixed flocks. 20 The distribution of green-headed tanagers 

 is comparatively restricted. They are confined to the wooded regions 



20 As the social behavior and social relationships of the green-headed tanager 

 seem to be at least roughly similar to those of the plain-colored tanager, it may 

 be of interest to compare some of the other characters of the two species. 



Like the plain-colored tanager, the green-headed tanager seems to have lost 

 the warbling song patterns that are conspicuous in the signal repertories of 

 many less gregarious species of Tangara (Butler, 1894). 



Mitchell noted that most of the plumage of many of the green-headed tanagers 

 in the flocks she watched was "mottled and nondescript." The birds with such 

 plumage may have been young. Mitchell also cites Descourtilz (1856) who 

 thought that grecn-hcaded tanagers take three years to attain fully adult plumage. 

 None of the Central American species of Tangara seems to take as long to attain 

 fully adult plumage (Skutch, 1954). It is conceivable, therefore, that green- 

 headed tanagers retain their dull juvenile plumage for a relatively long period 

 of time as an adaptation to facilitate their role in mixed flocks. Their rela- 

 tively dull juvenile plumage may subserve the same function, in the same way, 

 as the dull neutral plumage of both adult and young plain-colored tanagers. 



The fully adult plumage of green-headed tanagers is even more interesting in 

 this connection. In an earlier discussion of the plain-colored tanager, quoted 

 above, it was suggested that the dull neutral coloration of this species is partly 

 a compromise. Such coloration may have been evolved because plain-colored 

 tanagers could not evolve plumage colors and patterns more like those of their 

 most common associates in mixed flocks, simply because the colors and patterns 

 of these associates are themselves very diverse. It is possible, however, that 

 green-headed tanagers have been able to achieve this difficult feat. The species 

 that associate with green-headed tanagers are also very diverse in color and 

 pattern ; but the coloration of adult green-headed tanagers is far from dull. 

 It includes areas of orange, yellow, two or three shades of green, bright blue, 

 violet, and black, arranged in such a way as to provide the maximum amount 

 of "flash." Many of the other species that encounter green-headed tanagers may 

 find some or all of the colors of their own plumages matched or nearly matched 

 in the colors of the plumage of adult green-headed tanagers. This would be 

 expected to render adult green-headed tanagers particularly attractive to a wide 

 variety of other species. 



The plumage of adult red-necked tanagers {Tangara cyanocephala) , which 

 occur in some of the same flocks as green-headed tanagers, is equally brilliant 

 and varied, including areas of bright red, bright green, blue, and black. 



It might be convenient to call plumages such as those of adult green-headed 

 tanagers and red-necked tanagers "bright neutral" plumages, in contrast to the 

 "dull neutral" plumages of plain-colored tanagers and young green-headed 

 tanagers. 



